Softness is considered quite important for sanitary tissue products as these products typically come into contact with delicate and possibly inflamed regions of the human body including nasal, oral and perineal regions. Softness of sanitary tissue products can often be improved by adopting a multi-ply construction in which, for example, a finished tissue product having a basis weight in the neighborhood of 20 or 40 pounds per 3000 square-foot ream is comprised of two or three plies of tissue, each having a basis weight of approximately 8 to 17 pounds per ream. However, in many cases, the ply bonding technology used to integrate the plies into a single sheet of tissue prevents the full potential of the multi-ply technique from being realized. In some cases, as when adhesive is used for ply bonding, the effect can be to harshen the sheet, forcing the designer to adapt a compromise between effective ply bonding and softness. In other cases, as when plies are joined by embossing them together, one side of the resulting embossed structure will often be considerably harsher than the other, again at least partially defeating the intent of adopting a multi-ply construction.
Often sanitary tissue products having commendable softness can be obtained by combining either separately embossed plies or embossed and less highly embossed (possibly unembossed) plies such that any points or protrusions created by embossing are inwardly directed toward the center of the resulting multi-ply structure. Using these techniques, tissue products having a velutinous or velvety surface feel can be obtained; as the technique, in effect, shields the harsh points in the interior of the sheet. However, a great deal of the potential gain in softness achievable by this technique can be lost in those cases in which the plies are joined by adhesive.
Accordingly, in some commercial embodiments of this technique, plies have been joined to each other by knurled ply-bonding which avoids both the potential harshness entailed by liberal use of adhesives as well as the asperities created when embossed points are not concealed within the sheet. In a typical spot glassining operation, the tissue is spot glassined as it is rewound into the form of a “log”13 tissue wound onto the core upon which it will be sold, but before the individual rolls have been cut from the log. Accordingly, the “log” is of about the same diameter as a finished roll but is several feet in length, often 10 or more. Typically, two knurled ply-bonding wheels are allocated for each finished roll to be cut from the log. In typical operations, most of the cylindrical face of the knurled ply-bonding wheel is employed in forming a line of spot glassining; and, so, use of these conventional knurled ply-bonding wheels typically results in a very good, tight ply bond along the two highly compressed lines created when the previously unbonded multi ply structure is passed between the knurled ply-bonding wheels and anvil roll. As it can be somewhat difficult to control precisely where the marks left by the knurled ply-bonding wheels will fall relative to the ends of finished rolls; heretofore, this process has, in many cases, left an unfortunate, non-symmetrical “railroad track” appearance on the roll which some consumers find aesthetically unappealing, particularly if the spot glassining lines are not centered on the sheet, rolls having somewhat un-centered spot glassining lines being more common than perfectly balanced rolls.
We have discovered that it is possible to conceal, obscure or disguise the spot glassining lines in a multi-ply tissue product. In one method, we accomplish this by using knurled ply-bonding wheels of rather greater thickness than normal having spicules projecting from the cylindrical face thereof arranged in a sinuous or meandering path on the cylindrical face of the knurled ply-bonding wheel. By use of this technique: a plurality of spot embosses can be formed joining the plies together; the “railroad track” appearance of conventional spot glassining can be obviated; and the spot glassined pattern concealed, disguised or obscured in the embossed pattern. In the preferred embodiments, the plies are glassined together at the point of many of the spot embosses forming a tenacious bond that is quite durable, making it possible to achieve effective ply-bonding with a very small number of glassined spot embosses which have the further benefit of not creating asperities on either side of the multi-ply tissue product as the glassined tissue areas recede into the tissue away, from both surfaces.
In an alternative spot glassining technology, the knurled ply-bonding wheel has projections shaped to avoid formation of sharp discontinuities at the edges of the spot glassined regions. In preferred embodiments, we use a generally cylindrical knurled ply-bonding wheel that has a slight barrel shape, the peripheral cylindrical face bowing outwardly a slight amount, perhaps 10 to 50 mils, the shoulders sloping inwardly at about 10° to 25°, with the emboss elements being figuratively formed by transverse cuts tangent to the cylindrical face made through the bowed face at an angle of between 15° and 65°, preferably about 20° to 50°, with respect to the axis of the cylinder. In practice, it is more expedient to form the elements by taking a cylindrical wheel, grinding or turning away about 10 to 50 mils of the shoulders at an angle around 10° to 20 from the axis to form the bowed face, then knurling grooves into the bowed face and finally grinding away the very tips of the knurls to leave a thin planar plateau. The resulting emboss elements have a plateau region which is from about 3 to 12 mils in width as measured in the direction perpendicular to the cut and having a length of between about 20 to about 70 mils in the direction parallel to the cut. The preferred area of the peak is about 50 to 1000 square mils. The shoulders of the emboss peak fall off at an angle between about 10° and 25° and widen toward the periphery of the lateral face of the knurled ply-bonding wheel. We have found that if the shoulders of the emboss glassining area fall off gradually, say under about 30°, preferably under 20° and most preferably under 15°, the formation of a visually distinct sharp edge can be avoided on the tissue greatly diminishing the visibility of the line of knurls. It is further preferable that the long axes of the plateaus form a helical angle with respect to the axis of the knurled ply-bonding wheel of between about 15° and 45°.
Ultra premium bath tissue has become an important segment of the bath tissue market. An increasing portion of the population prefers bath tissue which is thicker, heavier in weight and more opaque. And, as always, ever-increasing levels of softness are preferred. In North America, the overall bath tissue market has heretofore been largely dominated by either single ply, particularly in the case of through air dried products, or double ply bath tissue, while the European market has had many entrants with three or more plies, primarily in the stronger grades, preferred in parts of that market. However, even with two ply products, consumers often experience problems with ply separation leading to difficulties in removing the desired quantity of product from the roll.
We have discovered that a 3 ply bath tissue largely meeting these demands can be formulated by the process of embossing two plies of bath tissue basesheet together, and mechanically combining these two plies with a third generally planar, or less heavily embossed, backing ply by either of the above described spot glassining procedures which glassine the layers together either with a number of spot embosses lying on a meandering path obscured in the embossing pattern on the embossed sheets or with very narrow glassined regions with indistinct ends which are far less visible than more sharply defined spot embosses. Typically the glassined spot embosses will be confined to only a very small area of the overall surface of the tissue. By use of this technique, a plurality of spot embosses can be formed joining the plies together; the “railroad track” appearance of conventional spot glassining can be obviated; and the spot glassined pattern concealed, disguised or obscured in the embossed pattern. In the preferred embodiments, the plies are glassined together at the point of many of the spot embosses forming a tenacious bond that is quite durable. After the log is formed, the tissue is preferably tail sealed by folding the exterior tail of the tissue back upon the roll and joining the resulting folded tail structure to the underlying layer of tissue with a controlled penetration adhesive such that a folded double-thickness tail is provided to the consumer for starting the roll.
In those embodiments in which maximum softness is desired, the first two plies of bath tissue may be embossed together with a pattern comprising groups of large emboss elements interspersed among a plurality of smaller emboss elements, the plies may be separated, one of these plies displaced relative to the other such that the groups of large emboss elements partially overlap, the embossed plies being subsequently combined with a third generally planar backing ply to provide a sheet have greatly increased caliper capable of imparting a sense of improved protection and thickness. In many of these embodiments, the width of the embossing nip (in the MD) used may somewhat exceed the width of the embossing nip which would normally be used for embossing two comparable plies together as the process of separating the plies tends to soften the emboss definition. It is preferred that the emboss pattern have a combination of groups of large emboss elements interspersed in a plurality of micro emboss elements and the displacement between the two heavily embossed plies be selected such that the groups of large emboss elements partially overlap, imparting an exaggerated puffiness to the appearance of these emboss elements making them appear billowy as compared to conventional emboss elements.
We have found that, by proper choice of the emboss patterns, parameters and substrates, we can achieve extremely high levels of consumer acceptance without requiring use of very high levels of softeners, debonders, conditioners or lotions as found in some current ultra premium bath tissue products. Not only can this simplify the manufacturing process considerably while removing a significant item of expense, it can also obviate concerns due to presence of high levels of chemicals in such products.
To achieve the foregoing advantages and in accordance with the purpose of the invention as embodied and broadly described herein, there is provided, in one embodiment of the invention, a three-ply tissue product formed by embossing together two heavily embossed plies with a third ply which is, at most, lightly embossed. The two heavily embossed plies are formed by an embossing process in which the two plies are embossed together then optionally separated. One of the two plies is then displaced, preferably longitudinally, relative to the other such that the groups of large elements on the two highly embossed plies only partially overlap and the plies are bonded to the third ply to provide an ultra bulky, low sidedness, soft three ply tissue. Preferably, the embossed plies are provided with a reticulated, tessellated emboss pattern, forming a pattern of cells with at least some of the cells being partially filled with a macro signature emboss comprising a group of large emboss elements. More preferably, a large portion of the void or unembossed areas remaining in the cells are filled with a micro pattern, the height of the elements forming the micro pattern being no more than about 60% of the height of the predominant elements in the macro pattern. In the most preferred embodiments, the third ply constitutes a lightly embossed or unembossed backing sheet masking the projections from the innermost sheet of said first and second plies. Surprisingly high softness can be achieved using this construction without requiring extensive use of eucalyptus or ultra-premium quality fibers.
In accordance with another aspect of the present invention, there is provided a multi-ply tissue product formed by embossing a first ply with a second ply, the embossed plies having groups of large scale embosses, an embossed area of at least about 2%, preferably more than 4%, more preferably greater than 8%, then ply-bonding by spot glassining the embossed plies together with a backing ply covering the projecting emboss elements on the intermediate embossed ply to form a multi-ply tissue product, wherein the multi-ply tissue product exhibits a plurality of emboss elements, said multi-ply tissue comprising: an upper embossed ply bearing a plurality of groups of large emboss elements interspersed among a plurality of smaller emboss elements; an intermediate ply bearing a substantially similar emboss pattern to said upper ply, and a generally planar backing ply joined thereto, said three ply sheet of cellulosic bath tissue exhibiting: a basis weight of at least about 25 pounds per 3000 sq ft ream; an opacity of at least about 72; a caliper of at least about 4.2 mils per eight sheets per pound of basis weight; a geometric mean of the mean deviation in the mean coefficient of friction of no more than about 0.8; and a geometric mean modulus of less than about 60; and a geometric mean tensile strength of less than about 35 g/3″ per lb. of basis weight.
In accordance with another embodiment of this invention, there is provided a roll of 3-ply sheets of cellulosic bath tissue having 3 plies of tissue joined together with an exterior tail projecting from the roll, comprising: an upper embossed ply bearing emboss elements; an intermediate ply bearing a substantially similar emboss pattern to said upper ply and being mechanically joined to said upper embossed ply by an entanglement/glassined region coincident with at least some of said emboss elements; and a generally planar backing ply mechanically joined to said intermediate ply by an entanglement/glassined region extending over less than about 1% of the area of said sheet, more preferably less than 0.1% and most preferably under 0.05% of the area of said sheet, the exterior tail of said roll being folded and adhesively bonded to itself with controlled penetration at a first location overlapping the tucked in tail of the roll and to the underlying layer in said roll at a second location, the distance between the first location and the second location being less than the length of tissue in said tail between said first and second locations; a plurality of said three ply sheets of cellulosic bath tissue exhibiting: a basis weight of at least about 25 pounds per 3000 sq ft ream; an opacity of at least about 72; a caliper of at least about 4.2 mils per eight sheets per pound of basis weight; a geometric mean of the deviation in the coefficient of friction of no more than about 0.8; and a geometric mean modulus of less than about 60. This embodiment provides a 3 ply tissue which largely overcomes the major problems experienced with ply-bonding while avoiding the loss of softness attendant upon the use of large amounts of adhesive for ply-bonding.